1. Field of the Invention
The present invention relates to the manufacture of parts such as complex geometry metals vanes and shrouds according to the technique known as lost wax moulding.
2. Discussion of the Background
For the manufacture of vanes and shrouds for turbojet engines, such as rotor or stator parts, or structural parts according to this technique, a master pattern is prepared first of all, using wax or any other similar material easily disposable at a later stage. If necessary, several master patterns are gathered into a cluster. A ceramic mould is prepared around this master pattern by dipping in a first slip to form a first layer of material in contact with the surface thereof. The surface of said layer is reinforced by sanding, for easier bonding of the following layer, and the whole is dried, which compose respectively the stuccowork and drying operations. The dipping operation is then repeated in slips of possibly different compositions, an operation always associated with the successive stuccowork and drying operations. A ceramic shell formed of a plurality of layers is then provided. The slips are composed of particles of ceramic materials, notably flour, such as alumina, mullite, zircon or other, with a colloidal mineral binder and admixtures, if necessary, according to the rheology requested. These admixtures enable to control and to stabilise the characteristics of the different types of layers, while breaking free from the different physical-chemical characteristics of the raw materials forming the slips. They may be a wetting agent, a liquefier or a texturing agent relative, for the latter, to the thickness requested for the deposit.
The shell mould is then dewaxed, which is an operation thereby the material forming the original master pattern is disposed of. After disposing of the master pattern, a ceramic mould is obtained whereof the cavity reproduces all the details of the master pattern. The mould is then subjected to high temperature thermal treatment or “baked”, which confers the necessary mechanical properties thereto. The shell mould is thus ready for the manufacture of the metal part by casting.
After checking the shell mould for internal and external integrity, the following stage consists in casting a molten metal into the cavity of the mould, then in solidifying said metal therein. In the field of lost wax moulding, several solidification techniques are distinguished currently, hence several casting techniques, according to the nature of the alloy and to the expected properties of the part resulting from the casting operation. It may be a columnar structure oriented solidification (DS), a mono-crystalline structure oriented solidification (SX) or an equiaxed solidification (EX) respectively. Both first families of parts relate to superalloys for parts subjected to high loads, thermal as well as mechanical in the turbojet engine, such as HP turbine vanes.
After casting the alloy, the shell is broken by a shaking-out operation, the manufacture of the metal part is finished.
During the moulding stage, several types of shells may be used via several methods. Each shell should possess specific properties enabling the type of solidification desired.
For example, for equiaxed solidification, several different methods may be implemented one using an ethylsilicate-based binder, another using a colloidal silica-based binder. For oriented solidification, the shells may be realised out of different batches, silica-alumina, silica-zircon or silica based batches.
The first layer for each of these shells plays an essential part. It forms the interface between the shell mould and the cast alloy. It should, in the case of columnar or mono-crystalline structure oriented solidification, be non-reactive with the cast alloy. In the case of equiaxed solidification, it should enable equiaxed germination of the grains. Besides, the integrity of this contact layer determines the final quality of the cast part, in terms of surface condition in particular.
The first layer should indeed meet certain requirements in order to avoid defects such as loss of ceramic cohesion and surface defects.
Loss of contact layer cohesion before or during the casting, may generate detrimental marks on the parts.
Surface defects result from excessive microporosity of the contact layer which generates surpluses forming bulges on the parts.
Major surface defects often result from a surface capillary phenomenon at the interface between the wax master pattern and the first layer. After dipping the first layer, during sprinkling, the grits will form stacks, which exhibit numerous capillaries. Each one acts as a suction cup which causes a depression. The smaller the capillary, the greater the depression. This corresponds to insufficient thickness of the first layer. Depression promotes capillary rising of the slip towards the plaster and so, until the liquid column thus formed restores the differential pressure. This is followed by the formation of a recessed zone with a cavity leading to the formation of surface defects. This phenomenon is worsened by too thin a first layer.
Both these types of defect, major defects in foundry, are associated with contact layer intrinsic antagonistic characteristics. Indeed, to avoid loss of ceramic cohesion, the purpose is to obtain thin and even deposit of the first layer, whereas to avoid surface defects, the deposit of the first layer should be even, but thick.
The properties of the contact layer should therefore enable to find a compromise between said antagonistic characteristics, in order to break free from all defects on the parts.